This invention generally relates to metal cutting tools, and is specifically concerned with a tool assembly comprising the combination of a metal cutting insert, and a chipbreaker mechanism that automatically adjusts itself to effectively break metal chips of widely differing thicknesses that result from the metal cutting operation.
Metal cutting tool assemblies utilizing a metal cutting insert in combination with a chipbreaker are known in the prior art. The inserts are typically formed from very hard, wear resistant materials such as refractory coated cemented carbide materials. While such inserts may assume any one of a number of differing geometrical shapes, they all have at least one cutting edge for engaging and cutting a metal workpiece. In operation, the cutting insert is detachably mounted on the end of a tool holder, and its cutting edge is forcefully engaged against a metal workpiece that is rotated relative to the workpiece by a machine tool such as a lathe. As the cutting edge of the insert cuts the metal workpiece, a long streamer of metal, known in the art as a "chip," is created that slides across the cutting edge of the insert. If no means for breaking such chips into short segments is provided, the chips might coil beyond the physical boundaries of the machine tool or wrap around the workpiece and machine tool, damaging one or the other. In view of this, chipbreaking mechanisms were developed in the prior art to break such chips into short manageable lengths of under two inches.
Prior art chipbreakers fall into two general categories, including (1) chipbreakers that are mechanically separable from the cutting insert mounted on the tool holder, and (2) chipbreakers which are integrally formed as part of the insert itself. In instances where the chipbreaker is mechanically separate from the insert, the chipbreaker may provide a single groove or trough-shaped surface directly behind the cutting edge of the insert. In operation, the metal chip is deflected from the cutting surface of the insert into the surface defined by the groove of the chipbreaker, which in turn causes the chip to curl back toward the workpiece. Because the curling of the metal chip work-hardens it and causes it to become embrittled, the chip breaks shortly after curling. In some instances, the chip curls back onto the surface of the workpiece and then breaks. Under these circumstances, however, the insert design and orientation is such that the chip is directed toward a portion of the workpiece ahead of the insert such that the already machined surface is not marred by contact with the chip. Chipbreakers that are integrally formed into the cutting inserts themselves work on approximately the same principle. The chipbreaking surface of an integrally formed chipbreaker may include a first surface which tapers down from the cutting edge which is called a land angle trailing wall, a second flat surface that connects with the bottom trailing edge of the first wall known as the chip groove floor, and a third surface known as the back wall which tapers upwardly from the trailing edge of the chip groove floor.
One of the shortcomings associated with either type of chipbreaker is that the groove or trough that defines the chipbreaking surface is limited in the thicknesses of chips that it can effectively break. When the cutting insert is used to make finishing cuts on a metal workpiece, the resulting metal chips are thin and foil-like, and the groove or trough which defines the chipbreaking surface must define a relatively tight curvature for such thin chips to be effectively curled, work hardened, and broken. While such a tight-curvatured groove might be able to break chips of intermediate thickness, the excessive heat created on the groove as a result of the larger amounts of friction between the thicker chips and the specific points of the groove surface that engage these chips can cause localized melting to occur on the chipbreaker surface, which ultimately leads to cratering. Such a condition is known in the art as over-controlled chipbreaking, and should be avoided as it creates excessive wear and tear on the chipbreaker surface. If progressively thicker cuts are made with a chipbreaker intended to be used in conjunction with only finishing cuts, at some point the chips will become so thick that they simply slide completely over the trough defined by the chipbreaking surface, and are not effectively broken at all. Such a condition is known in the art as under-controlled chipbreaking. Even if the chipbreaking groove or trough is dimensioned to effectively break a fairly wide range of chips having intermediate thicknesses, no static groove or trough geometry has yet been developed which is capable of breaking the complete range of thin and thick metal chips created as a result of different cutting depths.
While it is possible to change the chipbreaker used in conjunction with a particular cutting insert to break up chips which are substantially thicker or thinner than the chips previously produced, the downtime of the cutting tool associated with changing a separable chipbreaker results in expensive losses in productivity, and is an inconvenience and burden on the tool operator. Of course, in the case where the chipbreaker is integrally formed in the insert itself, the insert must be replaced every time the tool operator must make cuts of a significantly different depth in the rotating workpiece. This likewise leads to unproductive downtime, and essentially the same amount of inconvenience and burden on the tool operator. These limitations of prior art chipbreakers are particularly troublesome in machining operations for low carbon steel, where the chip thicknesses can vary widely.
Clearly, what is needed is a metal cutting tool assembly having a chipbreaker which is capable of breaking metal chips having a broader range of thicknesses than has been accomplished heretofore in the prior art. Ideally, such a chipbreaker mechanism should be simple in construction, and readily adaptable for use with existing turret blocks or spindles in machine tools. Finally, such a chipbreaker should be capable of breaking up a wide variety of chips having different thicknesses without either under-control of the resulting chips (i.e., allowing the chips to attain lengths of over 2 inches), or over-control which would apply such a powerful bending force on the chips that excessive crater-forming frictional heat is generated on the chipbreaker surface.